The present invention relates to a chain saw guide bar.
Various chain saw guide bar constructions are known in the art. One known example is illustrated in FIG. 15 of the accompanying drawings. A nose assembly 72 replaceably connected by rivets to a guide bar body 71. More specifically, the guide bar body 71 has a pair of laterally spaced central tongues 73 with a recess 74 defined therebetween. The nose assembly 72 has a sprocket 76 rotatably sandwiched between two side plates 75 having recesses 77 defined in tail ends thereof and receiving the tongues 73 respectively therein. For interconnecting the guide bar body 71 and the nose assembly 72, the tail end of a spacer 78 is fitted into the recess 74, and the front end of the spacer 78 is inserted between the side plates 75, and the spacer 78 is secured by rivets 79 to the guide bar body 71 and the nose assembly 72.
The prior chain saw guide bar however has the following problems:
When a saw chain 80 trained around an outer peripheral surface of the guide bar rotates at a high speed in the direction of the arrow R, a portion of the chain 80 (below the guide bar in FIG. 15) tends to be lifted off the rail under centrifugal forces. When the lifted chain portion contacts the rail, it hits the rail strongly so that the rail will be worn rapidly. The position subjected to greatest wear is about 70 to 90 mm spaced from the tip end of the nose assembly 72. Where a rail 71A of the guide bar body 71 is in such position, the guide bar body 71 will be worn and damaged.
Generally, the guide bar body 71 has a service life about three times that of the nose assembly 72. Therefore, the guide bar body 71 is required to be replaced once while the nose assembly 72 is replaced three times. The nose assembly 72 is the component which wears at the greatest rate, and needs frequent replacements. If the rail 71A of the guide bar body 71 is also worn, then there will be a step produced between the guide bar body 71 and a new nose assembly 72 coupled thereto. When a portion of the chain 80, which is rotated at a high speed, is caught by the step, the chain 80 is quickly worn, and generates vibrations and noise. Consequently the new nose assembly 72, which has just replaced the old one, is liable to get damaged.
To prevent the chain 80 from hitting the rail 71A of the guide bar body 71, the nose assembly 72 would have to be elongated so that an abutting portion A of the guide bar body 71 and the nose assembly 72 would be positioned at a distance 90 mm or more away from the tip end of the nose assembly 72. However, if the nose assembly 72 were too long, an abutting portion of the tongues 73 and the nose assembly 72 would be subjected to a large bending moment when subjected to a large lateral load while cutting wood, with the result that the guide bar would be bent or broken at the abutting portion B.